This invention related generally to a method for determining the inflow performance of multilayer reservoirs with multiple completed intervals that have been completed into a single wellbore. More specifically, some embodiments of the invention relate to methods for determining the liquid holdup and liquid and gas velocities in such a production wellbore, although the invention may be applied to other types of flow systems.
Wells are generally drilled to recover natural deposits of hydrocarbons and other materials trapped in subterranean formations. As the well is drilled, casing is installed in the wellbore that both supports the walls of the drilled hole and provides a flow passage to the surface. Perforations are subsequently made in the casing which extend into the formations at selected locations so that well fluids may flow through the perforations and into the casing. FIG. 1 shows a cross section of a typical well 101 penetrating a formation 103. The well 101 is lined with a casing 102, and the casing 102 and the formation 103 include one or more perforated zones 111, 112, 113. Formation fluids may flow from the formation 103 into the wellbore 101, where they may be transported to the surface.
Each perforated zone 111, 112, 113 shown in FIG. 1 represents an area of the well that has been perforated, typically using shaped-charged explosives, so that the formation fluids may flow into the wellbore and be transported to the surface. Each zone 111, 112, 113 is located in an area where the formation is believed to contain desirable fluids, such as oil, gas, or other hydrocarbons.
In many cases, a tubing string is placed in the wellbore, and the formation fluids are transported to the surface through the tubing string rather than the casing. FIG. 2 shows a tubing string 201 placed in a well 101 so that the production fluid flows to the surface through the tubing 201. One or more packers 203 may be used to isolate the interval containing the production zones 111, 112, 113 in the multi-layer reservoir and to stabilize the production tubing 201 in the well 101. In some cases, this is done because other production zones exist above or below the production zones 111, 112, 113 shown in FIG. 2. The packer 203 seals the annulus between the tubing and the casing so that only the formation fluids from the desired production zones 111, 112, 113 may flow through the tubing 201. Additional strings of tubing may be installed for producing from other intervals.
After the well has been completed, production logging may be used to obtain in situ measurements of the nature and behavior of the fluids in or around the wellbore during production. The purpose of production logging is to analyze the flowing performance of a well, including the productivity of different production zones in the well. This enables the well operator to produce and obtain maximum production from the well in the most economical way possible.
FIG. 3 shows one example of how production well logging may be performed. The tubing string 201 in positioned such that the lower end of the tubing string is located above the uppermost production zone 111 in the reservoir. A production logging tool 301 is lowered into the well 101 and data are taken as the tool 301 is moved through a vertical region where the measurements are to be taken. In FIG. 3, the production logging tool 301 is positioned between the two uppermost production zones 111, 112, where it may be used to measure flow rates, pressure, temperature, fluid mixture density, and fractions of liquid and gas, among other parameters. By positioning the production logging tool 301 in the position shown in FIG. 3, it will measure the cumulative flow that results from the inflow from production zones 112 and 113. The production logging tool 301 is shown schematically, and the tool 301 in FIG. 3 does not represent the shape or relative size of an actual production logging tool. There are multiple types of production logging tools known in the art, such as spinners, pilot tubes and turbine meters, that are typically used in combination with other measurement probes, including optical scanning devices to measure the fractions of gas and liquid in well stream.
Using a production logging tool 301, the flow characteristics of the well fluid stream may be measured and recorded (or transmitted to the surface) at multiple points. For example, the production logging tool 301 may subsequently be moved to a position between the second and third production zones 112, 113. In such a position, the production logging tool 301 would measure and record (or transmit to the surface) data based on the inflow from the lowest production zone 113. In this manner, the inflow performance may be determined for each of the production zones 111, 112, 113 by measuring the composite flow characteristics as multiple locations.
It is noted that a multi-layer interval may contain more than two or three production zones. A production logging tool may be relocated between each zone so that data corresponding to the properties of the inflow from each zone may be determined.